1. Field of the Invention
The present invention relates to a patient positioning device and a patient positioning method. More particularly, the present invention relates to a patient positioning device and a patient positioning method, which are suitably employed in a particle beam treatment system for irradiating a charged particle beam (ion beam), such as a proton and a carbon ion, to a tumor for treatment.
2. Description of the Related Art
There is known a treatment method of setting an isocenter (irradiation target center) at a tumor, e.g., a cancer, in the body of a patient and irradiating an ion beam, such as a proton, to the tumor. An apparatus for use with such a treatment method comprises a charged particle beam generator, a beam transport system, and a rotating gantry. An ion beam accelerated by the charged particle beam generator reaches the rotating gantry through a first beam transport system, and is irradiated to the tumor from an irradiation nozzle after having passed through a second beam transport system provided in the rotating gantry.
In the apparatus thus constructed, the patient must be caused to lie in a proper position relative to the irradiation nozzle so that the ion beam is irradiated to only the isocenter without damaging normal cells. A patient positioning device for use with irradiation of the particle beam is a device for positioning a patient couch to make the patient lie in the proper position (see, e.g., Patent Reference 1, JP,A 2000-510023 (pages 27–31 and FIGS. 1, 6, 7A and 7B)). Particularly, in the case of irradiating the ion beam, for example, a proton beam, activation energy for the proton beam is selected so as to stop protons at the isocenter and apply most of the proton energy to only cells in the tumor, which is positioned at the isocenter, by utilizing a characteristic that most of the proton energy is released upon the stop of protons (this phenomenon is called “Brag peak”). Therefore, the alignment of the ion beam with the isocenter is very important.
In a known patient positioning device, to ensure accurate positioning of the patient relative to the irradiation nozzle, the position of the isocenter is decided beforehand relative to monuments (or landmarks, i.e., anatomical base points; for example, portions of the patient's skeleton), which are set in the patient body. Usually, the position of the isocenter including a diseased tissue, e.g., a tumor, is marked on a DRR (digitally reconstructed radiograph). Then, display images looking from other directions are edited as required.
In a state where the patient lies on a patient couch prior to the irradiation of a proton beam, an X-ray source is disposed on a path of the proton beam, and an X-ray receiver is disposed on the side opposed to the X-ray source with respect to the patient along the path of the proton beam. The X-ray receiver produces an X-ray image of the tumor and its surroundings in the patient body. On this occasion, in order to align the isocenter on a beam line, through which the proton beam passes in the irradiation nozzle, with the tumor, the direction in and the distance by which the patient couch is moved relative to the irradiation nozzle must be determined by employing the offset distance on an X-ray image from each of the particular monuments to the center of the X-ray beam and the offset distance on the DRR from the same particular monument to the isocenter. Positioning control of the patient couch is performed based on the thus-determined direction and distance of movement of the patient couch.